The present invention relates in general to brushless DC motors of the so-called pancake type designed primarily for electronic computers, entertainment and medical aid devices, and similar densely packed electronic products, and more particularly to axial air gap sub-fractional horsepower brushless DC motors having two phases, a disc rotor, surface-wound windings, and an integral dry circuitry.
Heretofore, prior art brushless DC motors have centered around radial air gap designs using a permanent magnet on the rotor and windings on the stator. The rotor may be rotating either inside or outside the stator.
Axial air gap brushless motors are considered to be limited to computer disc drive applications where the pancake shape is most desirable. These applications call for the motor to overcome a friction load at very constant speed, usually below 1,000 rpm. The rotating permanent magnet is usually secured inside a cup made of soft magnetic material and serving as the flux return path. The additional inertia of the cup assists filtering of the speed variations and therefore provides a desirable feature. However, the cup prevents the motor from being used in higher performance applications because of poor acceleration. Poor efficiency at higher speeds is a second drawback. The surface-wound windings are secured on thin disc-like annular backing plate of soft magnetic material which generates eddy-currents when the magnet rotates. Eddy-currents are an undesirable effect which, along with hysteresis losses, manifests itself in iron power losses. The mathematical expression of eddy-current losses is P=.sigma.f.sup.2 B.sup.2 where .sigma. is a coefficient depending on the magnetic quality of the steel and the thickness of the lamination, f is the number of flux changes per second and B is the flux density. As can be seen from the equation, at low speed, eddy-current related power losses are low. At high speed, eddy-current related power losses make this motor design particularly unattractive. Higher performance applications of brushless DC motors usually feature a radial, rather than an axial, design where standard well-known means of correcting these two drawbacks can be implemented.
An object of the present invention is to provide a brushless DC motor design for high performance applications which overcomes the above mentioned problems and still provides the pancake-like form arrangement. This form arrangement is very useful in today's densely packed electronic products used in computer peripherals, entertainment and medical aid devices, and the like. In addition to its useful shape, the pancake-like motor also provides high output power versus volume or mass.
Electric motors based on the axial concept and operating at high speed are already available in many versions. Some use an ironless disc armature and others use a disc magnet. U.S. Pat. No. 4,330,727 discloses a step motor having a disc magnet on the rotor and two groups of elementary magnetic circuits coupled with a coil on the stator. Such an arrangement could be provided with position sensing devices for controlling the commutation of current in the windings, but due to the generally high number of pole pairs of a stepper motor, this would be rather difficult. The angular displacement of the sensing element in the motor would be extremely critical since one has to create an accurate commutation angle for each magnetic phase and within each electrical cycle, in order to obtain a smoothly running brushless DC motor featuring the highest possible torque. The two groups of elementary magnetic circuits are ideal for creating detent torque in a step motor, but are not a desirable feature in a brushless DC motor which has to operate without cogging. U.S. Pat. No. 4,072,881 discloses an axial air gap brushless motor including a rotor assembly having a disc-like permanent magnet and a stator formed of two groups of armature coils located on opposite sides of the permanent magnet. The armature coils are dipped inside grooves formed on the inner surface of the backing plate made of soft ferrite material. As the preferred embodiment description explains, the dipping of the armature coils into the grooves by about a half of the thickness of the armature coil still results in some amount of cogging torque. Disc shaped silicon steel plates are then suggested to replace the soft ferrite backing material in order to eliminate the cogging and obtain a smooth rotational torque. However, this solution is not retained because of the eddy-current power loss drawback mentioned above.
Another object of the present invention is the provision of a brushless DC motor of an axial design built for high acceleration and high speed, which will overcome the problems of the computer disc drive brushless motor by exhibiting a disc like magnet rotating in an air gap formed by two surface wound stator assemblies placed on either side of the magnet. Hereby the inertia of the rotor is reduced to the minimum possible, allowing high acceleration. This overcomes the problems of the step motor design by featuring a low number of cycles per revolution, typically four, and by using surface wound stator assemblies which do not create any cogging torque. The advantages of the design combine high acceleration and the absence of cogging torque with low electrical and mechanical time constants and with a linear torque versus current characteristic, so that the motor will not be limited by magnetic saturation but only by its thermal capabilities. The winding arrangement in this motor allows placement of two magnetic phases with its windings and sensing element within a motor housing of very short length and using the space available in an optimum fashion. As to the problem of eddy-current power losses in the stationary backing plates, the present invention offers a twofold novel solution to create in a disc shaped plate made of soft magnetic material an electrical open circuit which prevents eddy-currents from circulating and which does not induce cogging torque.
Other objects, advantages and capabilities of the present invention will become apparent in the following detailed description, taken in conjunction with the accompanying drawing illustrating a preferred embodiment of the invention.